Question on Cam Gear Marks - UPDATE

I no longer believe there is any valve timeing issue. Why would you suggest it in lieu of the overall story? Im open...
 
sorry russ, i follow the story, but its evolved over many days, and if i spoke of something irrelevant, its my memory, not the story thats the problem. just trying to help...best wishes
 
Russ, You say you haven't actually checked valve timing....the way I see it, if you're a tooth off, 44 teeth on the camgear divided by 360 degrees means each tooth gives just over 8 degrees. That equates to 16 degrees at crankshaft. being one tooth off would also advance the dissy by the same 16point whatever degrees. Then, depending on exactly where the points plate in distributor is located, could easily make up the rest of the ign advance you're seeing. As others have said, you need to degree in the camshaft first. And yes, a vac gauge will show if your valve timing is out, a very handy diagnostic tool I reckon. Good luck, Brian
 
If you look in the plug hole watch the valve operation, as the exhaust just is about closed the intake will just start opening, when you have both valves equally just a little open the piston should be at tdc for most cams
 
Further to Kurt's advice, if you observe through No6 plug hole, the valve overlap will occur as No1 is at TDC on the firing stroke.
 
When I install the cam gear, I always check for "Split overlap". We were taught that in shop in high school. Some early engines, like water pumps, didn't have timing marks. You don't need a dial indicator to tell you , You have a problem.
 
Ol Ron, can you explain the term "split overlap"? I am not familiar with that term and how you check for it.

Thanks, Zeke
 
Cam overlap is the time when both the intake valve and the exhaust valve are open around TDC. Split overlap is where both valve are at the same height above the valve seat and should be at TDC. I think the way Ron is using it is check TDC against both of the valves being in the same position.

As Kurt posted above: ...as the exhaust just is about closed the intake will just start opening, when you have both valves equally just a little open the piston should be at tdc for most cams
 
Last edited:
I might be wrong but overlap is something associated with performance engines. When I observe a flathead with a stock cam the exhaust closes then the inlet starts to open. They are not both open at a similar height at the same time.

But the bottom line is that the point at which the exhaust has just closed and the inlet is just about to start to open is the point at which the piston should be at TDC.

For reference, No.6 valves behaving as described above will coincide with No.1 at TDC having just fired. (On the firing stroke).
 
Mart said:
I might be wrong but overlap is something associated with performance engines. When I observe a flathead with a stock cam the exhaust closes then the inlet starts to open. They are not both open at a similar height at the same time.

But the bottom line is that the point at which the exhaust has just closed and the inlet is just about to start to open is the point at which the piston should be at TDC.

For reference, No.6 valves behaving as described above will coincide with No.1 at TDC having just fired. (On the firing stroke).
Click to expand...

Yep, but that is what the term means. Guess you could call it "zero" overlap. :)
 
cas3 said:
sorry russ, i follow the story, but its evolved over many days, and if i spoke of something irrelevant, its my memory, not the story thats the problem. just trying to help...best wishes
Click to expand...

I welcome any and all advice. With the new thread, I know it's gotten hard to follow.

I think I have eliminated any valve timeing concerns. I have a Sun machine, but the RPM and dwell functions are inop.
 
Mart, i think im seeing what you described as to the cam check. With number one at TDC both valves are closed. At that time, number 6 exhaust is almost closed. Any clockwise rotation closes the 6 exhaust and begins opening 6 intake. This puts to rest any concern about valve timeing, right?

Terry, I checked the points, both sets are closing, so it is running on both sets. When I get it running again, I'll try to measure the dwell. What should dwell be on a helmet type dizzy? Also will see if my tightening up the advance springs have retarded the timeing any.

Not going to be able to run it again today. Was 108* yesterday and supposed to be 105* today. Can't run it without heating up the garage.
 
Dwell in the 34-36 degree range will be about right.



It finally cooled off here...last couple weeks have been pretty nice in 70's and 80's. I do hate the summer weather, especially when our 95-105 temps combine with high humidity here.



Damn, this engine is being a pain in the butt!


Terry
 
Russ, Please clarify "almost closed".

From memory, with a stock cam in a flatty, as you reach TDC, The exhaust will have just closed and the inlet will be about to start opening. If the exhaust is closing after TDC then the timing may be out.

There are 44 teeth on the cam gear, one tooth is 8.18 degrees. So if you were one tooth out, the overlap (rocking) of the valves would occur 16 crank degrees after or before TDC, depending on which way you were off.

In other words, it should be obvious if it is wrong.

If it isn't obviously wrong, it is probably right.

Mart.
 
Russ, you said in your earlier thread that this is an Isky 77B grind.
The specs for this cam state inlet opens 19* BTDC and ex closes 19* ATDC.
So, at exactly TDC, both valves should be slightly open should they not?
OK, you've located TDC. So you know where the crank is....now you need to degree in the cam so you know where it is.
And then, when cam timing is correct....since the distributor is driven off of the front of the camshaft, if timed up correctly on a timing fixture, the ignition timing will be right!

I still reckon you're one tooth off on the camgear/crankgear, that advances everything 16 point something degrees [crankshaft degrees]. Then, I dunno how your distributor is set up, but it could quite conceivably be advanced, adding up to the 25* you're seeing at idle.
 
Last edited:
Brian said:
Russ, you said in your earlier thread that this is an Isky 77B grind.
The specs for this cam state inlet opens 19* BTDC and ex closes 19* ATDC.
So, at exactly TDC, both valves should be slightly open should they not?
OK, you've located TDC. So you know where the crank is....now you need to degree in the cam so you know where it is.
Click to expand...

Ya Brian, I only know one way to do these cam deals, so difficult to relate to just sticking them in. I know it is common practice, but I'm not a fan of tearing engines back apart chasing issues, LOL. Besides if your looking to maximize performance it's just about a requirement.

I'm sure Russ thinks I sound like a broken record (and I could be dead wrong), but I find it difficult to attempt to troubleshoot a problem like this without 100% nailing down all the basics.
 
Russ/40 said:
I welcome any and all advice. With the new thread, I know it's gotten hard to follow.

I think I have eliminated any valve timeing concerns. I have a Sun machine, but the RPM and dwell functions are inop.
Click to expand...

The dwell on my Sun machine doesn't work so I hook up a separate dwell meter.
 
Mart said:
I might be wrong but overlap is something associated with performance engines. When I observe a flathead with a stock cam the exhaust closes then the inlet starts to open. They are not both open at a similar height at the same time.

But the bottom line is that the point at which the exhaust has just closed and the inlet is just about to start to open is the point at which the piston should be at TDC.

For reference, No.6 valves behaving as described above will coincide with No.1 at TDC having just fired. (On the firing stroke).
Click to expand...

Hey Mart, The intake opens just before the exhaust closes, so both valves are off their seats at the same time near TDC but only for a few degrees which is called overlap. The exhaust stroke is overlapping the intake stroke for a few degrees.
Good subject for a video on Mart's Garage.
Bill
 
You must log in or register to reply here.
Back
Top